Circuit breakers with thermomagnetic tripping means are well known in commercial and industrial applications. For example, U.S. Pat. No. 3,162,739 discloses a breaker which has a bimetallic strip element for thermal trip and an electromagnetic element for instantaneous trip. Thermomagnetic circuit breakers are commonly used in distribution panels to incorporate both techniques with the electromagnetic element responding instantaneously to large surges in current (e.g., short circuits) and the bimetallic strip element responding to less extreme, but longer-term over-current conditions. The thermal portion of the circuit breaker provides an “inverse time” response feature which provides faster or slower response for larger or smaller over currents respectively.
When a fault is detected (e.g., the breaker's current rating is exceeded), the electromagnet disposed around the load conductor attracts an armature linked to a tripbar which rotates to release stored energy within the trip unit which then trips a frame and opens the contacts to interrupt the circuit. Typically, in an electromagnet system, a spring is used to hold the armature until a sufficient in-rush of current occurs which creates a magnetic force in the electromagnet strong enough to compress the spring and pull the armature towards the electromagnet which trips the breaker. The initial length and force needed to compress the spring is calibrated to provide an amount of force sufficient to prevent movement of the armature until there is a large enough in-rush of current. Thus, what is needed are methods and apparatus for easily calibrating the thermomagnetic trip unit of a circuit breaker.